In a 5-l. round-bottomed flask fitted with an efficient reflux condenser are placed 2750 ml. of 95% ethanol and 609 g. (8.0 moles) of thiourea. The mixture is brought to the reflux temperature on a steam bath, and the refluxing solution is almost clear. The steam is turned off, and 751.5 g. (4.0 moles) of ethylene dibromide is added in one portion. Within 5 minutes a vigorous reaction (Note 1) ensues and ethylene diisothiuronium bromide separates from solution. The exothermic reaction is allowed to continue to completion without further application of heat. The isothiuronium salt is collected by filtration and dried. The salt melts with decomposition at 225–227°(Note 2) and weighs 1104 g. (81%).

Concentration of the filtrate to a volume of about 250 ml. and recrystallization from 95% ethanol of the crude isothiuronium salt which separates gives an additional 130 g. of material. The total yield of the salt is 1234 g. (90%).

A mixture of 255 g. (0.75 mole) of ethylene diisothiuronium bromide and 640 g. (9.7 moles) of 85% potassium hydroxide in 1360 ml. of water is placed in a 5-l. round-bottomed three-necked flask and boiled under reflux for 5 hours. Ammonia is evolved during the reflux period. The flask is then equipped with a separatory funnel, a gas-inlet tube, and a condenser set for steam distillation (Note 3). Nitrogen is admitted through the inlet tube, and a cooled solution of 415 ml. of sulfuric acid in 760 ml. of water is added dropwise (Note 4). The addition is continued until the reaction mixture becomes acid to Congo red paper, and then a 20% excess of acid is added. Approximately 725–850 ml. of the acid solution is required. The heat of neutralization is sufficient to distil part of the dithiol. At the end of the addition of the acid, the passage of nitrogen is discontinued and steam is admitted through the inlet tube. The steam distillation is continued until about 3 l. of distillate is collected. The oil is separated from the water in the distillate, which is then extracted with two 500-ml. portions of ether. The ether solution and the oil are dried separately over calcium chloride. After evaporation of the solvent, the residue is added to the oil and the crude product is fractionated through a 10-in. Vigreux column under reduced pressure in an atmosphere of nitrogen. The ethanedithiol boils at 63°/46 mm.(Note 5) and weighs 39–44 g. (55–62%); nD20 1.5589.

2. Notes

1.
The reaction may be so vigorous that external cooling is required. A cloth wet with ice water and applied to the flask is sufficient to control the reaction.

2.
The melting point seems to depend upon the rate of heating. Use of a Fisher-Johns melting-point apparatus gives a value of 240–242°.

3.
At this point the reaction must be carried out in a good hood. The vapors of ethanedithiol may cause severe headache and nausea.

4.
The alkaline solution should be at room temperature before the acidification is begun.

5.
In one run the checkers observed a boiling point of 69°/46 mm.

3. Discussion

Ethanedithiol has been prepared from ethylene dichloride2 or ethylene dibromide3 and alcoholic potassium hydrosulfide; from ethylene dibromide and alcoholic sodium hydrosulfide;4 from ethylene dichloride5 or ethylene dibromide6 and alcoholic sodium hydrosulfide under pressure; from ethylene dibromide and thiourea;7 and by the catalytic hydrogenation with cobalt trisulfide8 or nickel-on-kieselguhr9 of the mixture resulting from the reaction of ethylene and sulfur. The present method is a modification of one described by Mathias.7

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